A printing device for recording image data with a thermal head is advantageous in that the noise level of recording is much lower than the noise level of an impact type printing device. A thermal head is also potentially smaller and less costly to manufacture than an impact printer, an ink jet printing device, or a laser-operated electrostatic printing device.
In a printing device with a thermal head, the energy level of thermal pulses outputted by heat generating elements can be varied. In this manner, a thermal printing device can express gradations in image tone by varying patterns of dots produced by a thermal color-developing recording sheet or an electrical heat-generating sheet.
FIG. 20 shows a theoretical arrangement of a printing device using an electrical heat-generating sheet. The electrical heat-generating sheet 1 comprises a resistance layer 2, a supporting layer 3, and an ink layer 4. The supporting layer 3 may also serve as the resistance layer 2 as the case may be.
A recording electrode 5 and common electrodes 6 are brought into contact with the sheet 1. When a voltage having a pulse width selected according to a desired recording pattern is applied to the electrode 5 by a signal voltage generating section 7, current flows to the common electrodes 6 through the resistance layer 2. In this operation, the current generates Joule heat that is conducted through the supporting layer 3 to the heat-molten ink layer 4 to melt the ink therein, and the molten ink is transferred as a dot onto a recording sheet 8.
In the printing device, the pulse width of the pulse applied to the recording electrode 5 may be varied to vary the level of the resulting heat pulse. The size of the area in the ink layer 4 where the ink is made molten will then change in accordance with the applied thermal eneergy. In other words, the diameter of a printing dot can be changed by controlling the time width of the pulse applied to the recording electrode. Accordingly, various gradations in dot size can be obtained by controlling the pulse width as described above.
FIG. 21 shows a printing device disclosed in Japanese Patent Application (OPI) No. 154772/1985 (the term "OPI" as used herein means "an unexamined published application").
In the printing device, in response to a request signal 23, an image signal generating section 10 outputs image data 21 comprising discrete picture elements wherein each picture element is represented by n bits. The outputted image data is applied through a tri-state gate 11 to a memory 12 and is stored in the memory. Stored data corresponding to l picture elements is read out, of the memory 12 and each picture element read out comprises n bits that are converted into m bit data by a gamma conversion table 13. The m bit data is suitable for controlling thermal printing elements. The m.times.l bits of data are transferred to a shift register circuit 15 including l m-bit shift registers.
The data outputted by the shift register circuit 15 are latched by a counter circuit 16 with the aid of a control signal 27. The counter circuit 16 has l counter units of m bits each, and provides a carry signal 30 depending on the data. A set signal 29 and a carry signal 30 sets and resets an l-bit flip-flop 17, respectively, so that l pulse width signals 31 are obtained for the data. During this operation, the next l.times.m data are transferred to the shift register circuit 15.
In the conventional printing device, one recording unit corresponding to the number of dots that can be printed in one recording operation is small, being only for l picture elements. Therefore, the technical concept of the conventional printing device is applicable to a serial printer having a recording unit in the order of thirty-two dots corresponding to thirty-two picture elements. The application of this technical concept to a line printer having a corresponding recording unit in the range of 1728 (54 characters) dots to 3072 dots (96 characters) per line is not practical, because a very large and complicated circuit would have to be provided to individually control the many heat generating elements of the thermal load.
A recent line-type thermal head has been designed to group the heat generating elements into blocks wherein one block corresponds to several hundred heat generating elements, and a printing control pulse is inputted for every block. If the aforementioned technical concept is applied to this line-type thermal head, then the resultant printing device is still very complex in construction and in circuitry, and the realization of an effective line-type thermal printing device is not practical.